Integrated circuits (ICs) include a multitude of transistors formed on a semiconductor substrate. Transistors, such as, metal oxide semiconductor field effect transistors (MOSFETs), are generally built on the top surface of a bulk substrate. The substrate is doped to form impurity diffusion layers (i.e. source and drain regions). A conductive layer is situated between the source and drain regions; the conductive layer operates as a gate for the transistor. The gate controls current in a channel between the source and the drain regions.
Accurate determination of the effective channel length becomes ever more important for device miniaturization, optimization, and modeling. Conventional efforts to find a simple and accurate method to determine or extract the channel length of a MOSFET device are based upon measurements of resistance, capacitance, and interface states. However, in order to use capacitance and interface related methods, higher resolution equipment is required to measure gate capacitance in the range of femto (10.sub.-15) farads and detect substrate current down to the level of pico (10.sub.-12) amperes, especially as gate lengths approach distances of less than 70 nm. As such, capacitance and interface related methods are not recommended for routine monitoring in a mass production environment.
Compared to capacitance and interface related methods, channel length extraction algorithms based on resistance are much simpler. Resistance related methods such as "paired V.sub.G (gate voltage)" and "shift and ratio" are used conventionally in practical applications. Nevertheless, the charge sharing effect by the source/drain depletion, gate bias dependence of the source/drain resistance, and the non-linearity problem inherent to proper calculation of V.sub.T (threshold voltage) make accurate determination of deep sub-micrometer channel length difficult using resistance related methods.
Thus, there is a need for a method for accurate channel-length extraction. Further, there is a need for a method of accurate channel-length extraction for use in mass IC fabrication processes. Even further, there is a need for a simple and accurate measurement of transistor channel length.